As infants acquire the ambient language, they become attuned to its articulatory properties and to how they are harnessed for phonological functions. This project's long-term goal is to understand the way in which native language experience comes to shape speech perception and production. How do infants, who can acquire any human language, become native speaker-listeners of the ambient adult language? Language-specific constraints on adults' speech perception and production, and their emergence in infants, provide a window on this process, which we examine in studies of language-specific effects in young children and adults. Our first specific aim is to examine how perceptual experience with the ambient language lays the foundation for language-specific phonological development. Previously, we developed a theoretical account of native phonological constraints on perception of nonnative phonetic information, referred to as the Perceptual Assimilation Model (PAM). PAM offers predictions of infants' and adults' discrimination of nonnative contrasts, which have been upheld for adults from several language environments, as well as for development of infants' perception of nonnative contrasts. In the next grant period, we test the hypothesis that perceptual assimilation of nonnative contrasts is predicted by articulatory factors, in studies comparing American English, Danish, French, Italian, and Korean listeners. We investigate the neural basis of language-specific perceptual attunement with fMRI studies. Also, our research on early attunement is extended to test the perception of articulatory aspects of nonnative speech by preschoolers and infants. The second specific aim is to characterize development of language-specific tendencies in production specifically in the phonetic properties of infant vocalizations. We extend our investigation of phonetic differences in the babbling of English-, French-, and Mandarin-learning infants during the second half-year, testing hypotheses about the emergence of language-specific effects in infants' productions of consonants, vowels, and syllabic organization. We also examine the link between perception and production in studies on infant imitation of speech and nonspeech vocal articulatory gestures. Phonological development research has important implications for early intervention on developmental speech and language deficits. Phonological disorders constitute a large proportion of speech therapy caseloads. Also, research suggests impaired phonological awareness is a primary cause of reading deficits in children. Thus, knowledge about normal phonological development can provide insights into developmental reading disabilities. Finally, this research is relevant to understanding second language learning, an issue of fundamental importance in our increasingly multilingual and multicultural society.